This invention relates generally to radio frequency (RF) oscillators and more particularly to RF oscillators which include voltage controlled variable capacitance devices for electronically tuning such oscillators.
As is known in the art, one type of RF oscillator includes a coaxial transmission line cavity having a negative resistance diode, such as an IMPATT diode, disposed at one end thereof and a load coupled to the other end thereof. A voltage controlled variable capacitance device, commonly referred to as a varactor, is capacitively coupled to the center conductor of the coaxial transmission line cavity to electrically vary the capacitance of such cavity and thereby electrically tune such cavity. Also included may be tuning stubs mounted about the center conductor and disposed between the negative resistance diode and the load to provide frequency tuning and impedance matching between the negative resistance diode and the load. In one such oscillator, the slugs are movable along the longitudinal axis of the coaxial transmission line and are so moved until the proper frequency and impedance matching are obtained. Then such slugs are locked into position. One locking technique used for this purpose is to provide each tuning slug with a screw which passes through a slot in the wall of the outer conductor of the coaxial transmission line into a hole tapped into the side of the tuning slug. When the screw is in a loosened condition, the slug is free to slide and, when the desired position is obtained, the screw is tightened thereby securing the slug in position relative to the outer conductor. While such technique may be satisfactory in some applications, in other applications as where the oscillator may experience high levels of mechanical vibration and shock the screws tend to loosen and the position of the slug thereby changes with the result that the negative resistance diode and the load are no longer impedance matched.
Further, when the RF oscillator is to be used in such a high mechanical vibration, high shock environment, it is necessary that the varactor also be securely mounted within the cavity. Further, the varactor must be mounted so that not only will it be able to withstand high levels of mechanical shock and vibration, but also so that proper direct current control signals may be applied to it with maximum isolation between the RF energy within the cavity and such control signals.